Many battery powered products require the use of multiple cell battery packs with the cells connected in series to achieve a high enough voltage for proper operation. The rechargeable type of battery packs need to be recharged periodically so that the powered operation can continue. And, special care needs to be taken to avoid damage to the battery cells during charging and operation. Two typical damages to battery cells are overcharging and deep-discharging.
Overcharging may cause severe damages to battery cells, and may even become safety concerns. Overcharging lithium-ion or lithium polymer batteries, for example, may cause thermal runaway, and the high temperature developed may lead to cell rupture. Fire hazards have been reported during charging as extreme cases. Therefore, much attention has been paid to battery overcharging and solutions have been designed to avoid battery damage and safety issues. The typical charging system developed applies one charger to charge all cells connected in series in a battery pack. Since all cells are not manufactured the same, some cells may charge faster than others. As such, charging a battery pack with a single charger charging a plurality of cells can lead to overcharging sonic of the cells, U.S. Pat. No. 4,079,303, issued to Cox on Mar. 14, 1978, discloses a two step charging system to charge all battery cells connected in series at an initial charging rate to a predetermined voltage threshold, followed by an equalization procedure for charging each of the cells with controlled and equal voltage to fully charged state. The disadvantage of such a system is that the equalization phase is based on a conservative estimate of a predetermined charging voltage threshold. And the charging process can be time consuming when the battery pack contains many cells.
A common practice of using a single charger to charge a battery pack of a plurality of cells is to use a shunt circuit to bypass the charging circuit of each individual cell when it is fully charged to avoid overcharging and over heat. U.S. Pat. No. 6,388,424B1, issued to Hidaka et al on May 14, 2002, teaches a system for charging a plurality of lithium-ion battery cells connected in series. And, each of the cells has a shunt circuit connected in parallel with the cell. A comparator compares the charging voltage of each cell with a reference voltage. When the charging voltage is higher than the reference voltage, a switch is activated to direct the electricity to the shunt circuit for the cell. Although the invention is trying to supply the surplus energy to the next cell in line, this system inevitably causes energy waste for charging which is not friendly to the environment.
Another damage that can happen to battery cells is deep-discharging. And this is especially true for lithium-ion and lithium polymer batteries. When a charged battery pack of a plurality of cells is connected to a load, each cell is gradually discharged, and the cell voltage declines. When a cell is discharged below a defined low voltage threshold, further discharging may damage it. After that, the cell may have degraded storage capacity. For example, a certain lithium-ion battery cell of the LiFePO4 variety should not be discharged below 2.5 volts to avoid deep-discharging damage. It is important, therefore, that care is taken to remove the battery load before the low cell voltage limit is reached.
Therefore, it is important to manage individual cells in a battery pack to effectively avoid over charging or deep-discharging of the each cell. For a battery pack of a plurality of cells, each cell can behave differently due to variations of manufacturing and parts supply. It is apparently advantageous to measure the voltage of each of the cells due to the uncertainties. In this way, when the voltage of any one of the plurality of the cells reaches a predetermined low voltage limit, a decision is made to unload the battery pack. And the battery pack is effectively protected. It is also advantageous to individually manage the charge of each of the plurality of cells in a battery pack so that no one cell is overcharged.
Therefore, there is a need for a simple and inexpensive way to manage each cell in a battery pack containing a plurality of cells for the best battery protection and extended battery life.